The present invention relates to a metal halide lamp with a ceramic arc tube.
In a metal halide lamp having a ceramic arc tube, a material of the arc tube and a filled metal react less than those in a metal halide lamp having a quartz arc tube, which has generally been used so far. Therefore, a stable lifetime property is expected.
Conventionally, this kind of metal halide lamp having an arc tube that is a translucent alumina tube closed with an insulating ceramic cap or a conductive cap at both ends is known (see, for example, JP No. 62-283543 A).
Another known metal halide lamp is disclosed in, for example, JP No. 6-196131A. In this metal halide lamp, both end portions of a ceramic arc tube have a smaller diameter than that of the central portion, electrically conductive lead-wires having an electrode at their tips are inserted at the both end portions, and the gap between the end portions of the arc tube and the conductive lead-wire is sealed with a sealing material
Such conventional metal halide lamps using ceramic arc tubes have a well-known configuration in which high thermal resistance of a ceramic is used in order to enhance the lamp efficiency, thereby increasing the tube-wall load of the arc tube (lamp power per surface area of the entire arc tube) compared with metal halide lamps having a quartz arc tube.
As shown in FIG. 5, these metal halide lamps generally have electrodes having a structure in which the end face of an electrode coil 55 is positioned in the same plane as an electrode bar 54 (hereinafter, a flush structure will be referred to). Furthermore, there has been no detailed research about the relationship between the electrode structure and the occurrence of lamp flickering or the lifetime of lamps.
When compared with the metal halide lamp using a quartz arc tube, in the above-mentioned conventional metal halide lamp using a ceramic arc tube, it is possible to increase the tube-wall load of the arc tube and to realize high efficiency and high color rendition. On the other hand, since the temperature inside the arc tube is high and the electrode temperature is high, the deformation at the tip of the electrode is increased. As a result, the arc length is increased, which may lead to an increase in the lamp voltage, thus causing an early lamp break-off.
In the conventional metal halide lamp using ceramic arc tube, the shape of the tip of the electrode was optimized by employing the flush-structured electrode so as to reduce the increase in the arc length due to the deformation of the electrode tip, and suppress the lamp break-off.
On the other hand, in the conventional metal halide lamp having the flush-structured electrode, the rate of occurrence of lamp flickering is increased due to the movement of a discharge luminescent spot on the electrode coil. Furthermore, the discharge on the electrode coil is likely to occur, which may raise the temperature of the electrode coil locally. As a result, the evaporation of the electrode coil materials during the lifetime is increased, which may cause problems of blackening of the arc tube or reduction of the luminous flux maintenance factor.
It is an object of the present invention to provide a metal halide lamp in which the lamp flickering is reduced, the luminous flux maintenance factor during the lifetime is radically improved, and the lamp break-off is suppressed.
In order to achieve the above-mentioned objects, the metal halide lamp according to the present invention includes an arc tube of translucent ceramic in which a metal halide is filled; and a pair of electrodes provided in the arc tube, the electrode having an electrode bar and an electrode coil; wherein the following relationship is satisfied:
0.00056xc3x97W+0.061xe2x89xa6xcex1xe2x89xa60.0056xc3x97W+1.61
where xcex1 (in mm) is a length of the portion of the electrode bar protruding from the end face of the electrode coil and W (in Watt) is the lamp power.
According to such a configuration, since the discharge luminescent spot is stable at the tip of the electrode bar and heat is released effectively by the electrode coil at the tip of the electrode bar, the increase in the lamp voltage and blackening of the arc tube are suppressed. Therefore, it is possible to provide a metal halide lamp with less lamp flickering, an improved flux maintenance factor and low possibility of lamp break-off.
It is preferable in the above-mentioned metal halide lamp that the ratio of sodium iodide with respect to the total amount of the metal halide is 10 wt % or more.
According to such a configuration, since the temperature inside the arc tube is reduced and thus the electrode temperature is reduced, the increase in the lamp voltage can be suppressed more effectively.